This weekend there was a problem (http://googleblog.blogspot.com/
2009/01/this-site-may-harm-your-computer-on.html) with the
implementation of Google's malware filters. Gmail's spam engine uses
those filters (among hundreds of other signals) to help protect our
users from malware, and so between 6:00 a.m. PST and 8:00 a.m. PST, we
mistakenly sent some legitimate mail to people's spam folders.

We've rolled out a fix that has restored these messages to most
people's inboxes, though to be on the safe side we'd still recommend
that you check your spam folder if there was a critical message you
expected to receive between 6am and 8am PST on Saturday.